Speedometer Gear Calculator
Calculate the correct speedometer gear size for accurate speed readings
Speedometer Gear Calculator
What do you want to calculate?
Common ratios: 2.73, 3.08, 3.23, 3.55, 3.73, 4.10, 4.56
Number of teeth on the driving gear (typically 15-50)
Calculation Results
Speedometer Accuracy
Technical Details
Common Gear Sizes
Drive Gear Teeth
Driven Gear Teeth
Quick Tips
Larger Tires
Need fewer driven gear teeth for accurate speedometer
Gear Ratio
Higher ratio = more driven teeth relative to drive teeth
Accuracy
Within 2% error is considered excellent
Installation
Round to nearest available gear size
Understanding Speedometer Gears
What is a Speedometer Gear?
A speedometer gear is a mechanical component that translates the rotation of your vehicle's transmission output shaft into speedometer readings. The gear ratio between the drive and driven gears determines how accurately your speedometer displays your actual speed.
Key Calculations
Revolutions per mile: R = 20,168 ÷ Tire Diameter
Gear ratio: G = (Axle Ratio × R) ÷ 1,001
Driven teeth: Drive Teeth × Gear Ratio
Why It Matters
- •Accurate speed readings for legal compliance
- •Proper odometer function for maintenance tracking
- •Correct operation of speed-sensitive systems
When to Replace
Tire Size Change
When installing larger or smaller tires, the speedometer gear must be changed to maintain accuracy. Even a 1-inch diameter change can cause significant speedometer error.
Transmission Swap
Different transmissions may have different output shaft speeds, requiring a different speedometer gear to maintain proper calibration.
Axle Ratio Change
Changing the differential gear ratio affects the relationship between wheel speed and transmission output speed, requiring speedometer adjustment.
Frequently Asked Questions
What happens if my speedometer gear is wrong?
An incorrect speedometer gear will cause your speedometer to read higher or lower than your actual speed. This can lead to speeding tickets, inaccurate odometer readings, and improper operation of cruise control and other speed-sensitive systems.
How accurate should my speedometer be?
Ideally, your speedometer should be within 2% of your actual speed. Most regulations allow for some error, but for safety and legal compliance, accuracy within 5% is generally acceptable.
Can I use this calculator for metric measurements?
Yes! The calculator accepts tire diameters in inches, millimeters, or centimeters. The calculations are automatically converted to the standard formula using inches, as this is the traditional measurement system for speedometer gear calculations.